This invention relates to unique coherent shapes or elements of silicon carbide in which silicon carbide is the predominant component, and method of producing them. The term "shape" is used in the dictionary sense of "a thing having a particular form or figure." The term "element" is used to mean the same thing, the term "element" being a more commonly used one than the term shape. The invention will be described in conjunction with the production of heater elements and igniters.
Silicon carbide rods for resistance elements are available commercially, as are silicon carbide igniters. One method for making them is by forming a rod of silicon carbide, silicon and carbon, consolidated by pressure, in an unfired state. The rods are packed in carbon and put in a furnace for sintering to lower the resistance of the rod. An example of such a method is set out in U.S. Pat. No. 2,897,572. The process requires a time span measured in hours, is cumbersome, expensive and not adapted to the production of light and complicated shapes of precisely controlled resistance. Silicon carbide igniters known heretofore have been cut from rods or tubes of silicon carbide made in the conventional way.
The preferred embodiments of shapes of this invention for use as heater elements to be operated at relatively low temperatures are made of doped silicon carbide and an electrical resistivity modifying agent, generally an electrically insulative refractory material such as aluminum oxide, molybdenum disilicide, magnesium fluoride, magnesium chloride, or magnesium titanate or some combination thereof. In the production of the preferred embodiment of igniters, doped silicon carbide powder is admixed for the hot zone of the igniter with silicon powder which is later converted to silicon carbide, and for a contact zone, with aluminum powder and silicon powder.
In any event, when the shape is formed by the preferred method of this invention in which a laser is used, the shape is characterized by having a core with a distinctive valley or grooves in it, the core being bonded by regrowth of silicon carbide, and a distinctly delineated rind adhered to the core but removable therefrom, the rind being bonded by the formation of quartz.
The term "doped" as applied to silicon carbide is used herein in the commercially accepted sense in the semi-conductor art to mean treated with a very small amount (less than 1% by weight) of a material which renders the silicon carbide semi-conductive. The silicon carbide can either be of the type known commercially as green grit, with an n-type dopant such as nitrogen, or black grit, with a p-type dopant such as aluminum.
The shapes or elements of this invention may be non-linear of complicated configuration or varying cross-section, or both, and with various reaches of the top surface lying in different planes or in substantially the same plane. In the latter case, they can be used in applications for which silicon carbide elements have never heretofore been used, such as range burner elements. In addition, depending upon the amount and character of electrical resistivity modifying agents in the composition, the shapes or elements lend themselves to use as thermal sensors or thermistors. They may be made either uniform or non-uniform in composition through their length.
One of the objects of this invention is to provide silicon carbide shapes of a character heretofore unknown.
Another object is to provide such shapes in configurations and for purposes not heretofore possible.
Another object is to provide a method for producing such shapes quickly, easily, and economically.
Other objects will become apparent to those skilled in the art in the light of the following description and accompanying drawings.